In recent optical communication systems, a wavelength division multiplexing (WDM) transmission technique has been increasingly important. In the WDM transmission technique, strict adjustment of the wavelength dependence of dispersion of an optical fiber is strongly desired because it is important to provide uniform transmission properties for each wavelength by decreasing the differences in chromatic dispersion in a wavelength band that is used. Optical fibers in which the wavelength dependence of dispersion is adjusted are, for example, a dispersion shifted optical fiber, a non-zero dispersion shifted optical fiber, a dispersion flattened optical fiber, and a dispersion compensating optical fiber. These optical fibers have a complicated refractive index profile in which a plurality of maximum points and a plurality of minimum points are present.
In order to obtain desired dispersion properties in such an optical fiber as described above, it is important to precisely form the refractive index profile. A method suitable to precisely form the refractive index profile is, for example, an inside vapor phase deposition method such as an MCVD (Modified chemical vapor deposition) method or a PCVD (Plasma-activated chemical vapor deposition) method. As described in “Optical Fiber Communications, International Edition 1991”, pp. 66 to 67, published by McGraw-Hill Book Co., the MCVD method has been widely used because an optical fiber preform having high quality can be obtained relatively easily with it.
Also, U.S. Pat. No. 4,820,322 discloses a method of manufacturing an optical fiber, in which method a glass rod formed by collapsing a tube that is provided with glass deposited by an MCVD method is drawn together with an outer portion that is a part of a cladding, while the glass rod and outer portion are being unified together.
Incidentally, in order to obtain a high quality optical fiber, it is important to prevent hydroxyl groups (OH groups) from entering the glass in a manufacturing process. Since an optical fiber in which the wavelength dependence of dispersion is adjusted has a complicated refractive index profile, it is formed by a complicated manufacturing process, and as a result, OH groups are liable to enter the glass. If OH groups are present in a light propagation region of a fiber, light is absorbed by the OH groups, and accordingly the transmission loss increases.